A typical device in an emitter-switching configuration is formed by a bipolar power transistor for high voltage use, and by a field-effect transistor for high frequency and low voltage use. These two transistors are connected to one another with the emitter terminal of the former connected to the drain terminal of the latter. The former component is selected to withstand high voltages, and the latter is selected to avoid the poor performance of the former during switching while in operation. A switch element of another type, for example, a bipolar transistor for high frequency and low voltage use may be used instead of the field-effect transistor.
Typically, the high-voltage transistor has an open-emitter collector-base breakdown voltage (BVcbo) which may be up to 2000V, whereas the low-voltage transistor has a breakdown voltage of about 60V. This configuration is advantageously used in circuits for controlling the switching of a load in which the switching speed is important. The operating frequency in this type of configuration may be several hundred kilohertz.
Various methods are known for operating a device in an emitter-switching configuration. Two of the most often used methods are shown in FIGS. 1 and 2. Referring to FIG. 1, the base terminal of the bipolar power transistor Q and the gate terminal of the field-effect transistor M are driven by the same input signal Vin. The two terminals are separated by a resistor R1. Referring now to FIG. 2, only the gate terminal of the field-effect transistor M is driven by the input signal Vin, while the base terminal of the bipolar power transistor Q is connected to a constant voltage supply Vcc. This constant voltage supply Vcc, in many cases, is different from the voltage supply Vdd of the load RL.
The system of FIG. 1 is used without problems in all cases in which the driving signal Vin is a rectangular wave. However, the system is not very efficient if the waveform of the driving signal is sinusoidal. The system of FIG. 2 does not have this problem, but may require a voltage supply Vcc separate from the main supply Vdd for biasing the base of the bipolar power transistor. In certain applications, this additional supply has to be at a voltage higher than the main voltage. This creates circuit design problems, and complicates the circuit.
A sinusoidal input voltage signal Vin, as shown in FIG. 3a), and an inductive load RL are considered with reference to the system of FIG. 1.
The base current Ib is also sinusoidal, as shown in FIG. 3b), and the collector current Ic has a triangular waveform as shown in FIG. 3c). The maximum value of the base current occurs not when the collector current is at a maximum, but at a moment at which the collector current has a value below the maximum value. In these operating conditions, since collector current Ic flowing through the bipolar power transistor Q increases as the base current Ib decreases, its operating point may move from the saturation region to the active region, as shown in FIG. 4. This drawing shows the characteristic curves of the collector current Ic as a function of the voltage Vcs. The voltage Vcs is between the collector of the bipolar transistor Q and the source terminal of the field-effect transistor M for various values of the base current Ib.
Two operating points, indicated a and b, are shown in FIG. 4 and correspond to two collector-current values Ic.sub.1, and Ic.sub.2, respectively. The former is on a characteristic curve corresponding to a base current Ibn, and is in the saturation region of this curve. The saturation region is a low voltage Vcs1 of the collector relative to the ground terminal. The latter is on a characteristic curve corresponding to a base current Ibn-1 less than the current Ibn. This is in the active region of this curve, that is, at a high voltage Vcs.sub.2 of the collector relative to ground.
The curve of the voltage Vcs between the collector and ground in the positive half-period of the input voltage Vin is shown in FIG. 3d). This method of driving the device in an emitter-switching configuration causes the bipolar power transistor Q to operate in the active region of its characteristic curve during part of the time during which it should function as a closed switch. This also causes an increase in the power dissipated Pdiss, as shown in FIG. 3e).
With the driving system of FIG. 2, the base current Ib also decreases as the collector current Ic increases during part of the time when the bipolar power transistor Q is conductive. However, the effects of this are negligible if the biasing voltage Vcc selected is significantly greater than the sum of the voltage drop Vbe between the base and the emitter of the bipolar transistor Q, and the voltage drop Vds between the drain and source terminals D and S of the field-effect transistor M.